A previous back surface electrode-type solar cell is schematically shown in FIG. 15 as a cross sectional view. The back surface electrode-type solar cell 110 produced by a previous art will be described by referring to FIG. 15. On the light-receiving surface side of an N-type silicon substrate 113, a rugged shape 114 and an FSF (Front Surface Field) layer 115, which is an N-type diffusion layer, are formed. On the rugged shape 114, a dielectric passivation layer (a surface passivation layer) 117 which contains silicon dioxide and an antireflective film 116 which contains silicon nitride are formed from the side of the N-type diffusion layer 113.
On the backside of the N-type silicon substrate 113, an oxide layer (the first backside passivation film) 119 is formed. In addition, on the backside of the N-type silicon substrate 113, N-type doped N-type diffusion layers 120 and P-type doped P-type diffusion layers 121 are alternately formed. On the N-type diffusion layer 120, an N-type metal contact 11 is formed; and on the P-type diffusion layer 121, a P-type metal contact 12 is formed. These contact electrodes, which are joined directly to the substrate itself, can function as finger electrodes for collecting current.
FIG. 19 is a top view schematically showing the appearance of the backside of the previous back surface electrode-type solar cell. As shown in FIG. 19, the back surface electrode-type solar cell is provided with a pair of bus bar electrodes (an N-type bus bar electrode 22, a P-type bus bar electrode 23) at the edge of the substrate for collecting current from finger electrodes (the N-type metal contact 11, the P-type metal contact 12). Although the electrodes nearest to the periphery of the substrate are depicted as N-type metal contact electrodes in FIG. 19, they may be P-type metal contact electrodes or electrodes of different type with each being P-type and N-type.
To improve the efficiency of the back surface electrode-type solar cell, full enlargement of the P-type diffusion layer, which is a power generation layer, can be expected to increase short circuit current. Accordingly, it is desirable to form the region of the P-type diffusion layer widely such as the ratio of the P-type diffusion layer and the N-type diffusion layer in a range of 80:20 to 90:10. When the area of contact between the substrate and the contact electrodes (hereinafter, also referred to as contact area) is decreased as possible, and the passivation regions are enlarged, increase of open circuit voltage can be expected. Accordingly, it is desirable to design the metal contact region as small as possible by making the contact electrodes in thin line shapes or dot shapes.
Patent Literature 1 discloses a back surface electrode-type solar cell in which the contact area of the electrodes and the substrate is suppressed to the lowest possible, and the passivation regions are enlarged by three steps of forming contact electrodes, covering the portion other than the contact electrodes with an insulation film, and forming a wiring electrode.
FIG. 16 is a top view schematically showing the appearance of the backside of the previous back surface electrode-type solar cell disclosed in Patent Literature 1. In the solar cell of Patent Literature 1, however, only one pair of bus bar electrodes (an N-type bus bar electrode 22, a P-type bus bar electrode 23) are formed at the periphery of the substrate (see FIG. 16). In this arrangement, the finger electrodes are long, and accordingly the wiring resistance becomes extremely large, which causes lowering of a fill factor. It is considered that this can be solved by designing the wiring electrodes (finger electrodes) to have enlarged cross-section or the finger to have shortened length.
FIG. 17 is a top view schematically showing the appearance of the backside of the previous back surface electrode-type solar cell disclosed in Patent Literature 2. As shown in FIG. 17, Patent Literature 2 discloses an electrode shape of a solar cell, for example, being provided with plural pairs of bus bar electrodes 30 so as to shorten the length of finger electrodes 41. In this arrangement, the finger length becomes L/3 based on the substrate length L, and the wiring resistance becomes one third compared to the case having a pair of bus bar electrodes. In this case, however, the bus bar electrodes are arranged at the periphery, and the bus bar electrodes at the periphery only collect current from the finger electrode on one side.